A low-numerical-aperture (NA) concept enables large-mode-area fiber with better single-mode operation ability, which is beneficial for transverse mode instability and nonlinear effects suppression. In this contribution, we reported a high-power fiber amplifier based on a piece of self-developed large-mode-area low-NA fiber with a core NA of 0.049 and a core/inner cladding diameter of 25/400 µm. The influence of the pump wavelength and fiber length on the power scaling potential of the fiber amplifier is systematically investigated. As a result, an output of 4.80 kW and a beam quality factor of ∼1.33 were finally obtained, which is the highest output power ever reported in a fiber amplifier exploiting the low-NA fiber. The results reveal that low-NA fibers have superiority in power scaling and beam quality maintenance at high power levels.

3 μm激光处于分子指纹区，在医疗外科、气体检测、**应用等领域都有重要的应用价值。Er³⁺∶ZBLAN光纤激光器具有效率高、可集成的优点，是3 μm激光的主要输出方式。本文从铒离子跃迁产生3 μm激光出发，围绕Er³⁺∶ZBLAN光纤激光器，介绍了3 μm激光产生的结构原理及能级系统，总结了实现该波段高功率连续输出和脉冲输出的技术方案和研究进展，重点介绍了基于不同材料可饱和吸收体的调Q和锁模激光器实验研究，并对目前实现3 μm波段高功率输出需要解决的问题进行了分析，最后对Er³⁺∶ZBLAN激光器的发展方向进行了展望。

A passively switchable erbium-doped fiber laser based on alcohol as the saturable absorber (SA) has been demonstrated. The SA is prepared by filling the gap between two optical patch cords with alcohol to form a sandwich structure. The modulation depth of the alcohol–SA is measured to be 6.4%. By appropriately adjusting the pump power and the polarization state in the cavity, three kinds of mode-locked pulse patterns can be achieved and switched, including bright pulse, bright/dark soliton pair, and dark pulse. These different soliton emissions all operate at the fundamental frequency state, with a repetition rate of 20.05 MHz and a central wavelength of ∼1563 nm. To the best of our knowledge, this is the first demonstration of a switchable soliton fiber laser using alcohol as the SA. The experimental results further indicate that organic liquid-like alcohol has great potential for constructing ultrafast lasers.

Although visible femtosecond lasers based on nonlinear frequency conversion of Ti:sapphire femtosecond oscillators or near-infrared ultrafast lasers have been well developed, limitations in terms of footprint, cost, and efficiency have called for alternative laser solutions. The fiber femtosecond mode-locked oscillator as an ideal solution has achieved great success in the 0.9 to 3.5 μm infrared wavelengths, but remains an outstanding challenge in the visible spectrum (390 to 780 nm). Here, we tackle this challenge by introducing a visible-wavelength mode-locked femtosecond fiber oscillator along with an amplifier. This fiber femtosecond oscillator emits red light at 635 nm, employs a figure-nine cavity configuration, applies a double-clad Pr^{3 +} -doped fluoride fiber as the visible gain medium, incorporates a visible-wavelength phase-biased nonlinear amplifying loop mirror (PB-NALM) for mode locking, and utilizes a pair of customized high-efficiency and high-groove-density diffraction gratings for dispersion management. Visible self-starting mode locking established by the PB-NALM directly yields red laser pulses with a minimum pulse duration of 196 fs and a repetition rate of 53.957 MHz from the oscillator. Precise control of the grating pair spacing can switch the pulse state from a dissipative soliton or a stretched-pulse soliton to a conventional soliton. In addition, a chirped-pulse amplification system built alongside the oscillator immensely boosts the laser performance, resulting in an average output power over 1 W, a pulse energy of 19.55 nJ, and a dechirped pulse duration of 230 fs. Our result represents a concrete step toward high-power femtosecond fiber lasers covering the visible spectral region and could have important applications in industrial processing, biomedicine, and scientific research.